The first amniotes evolved
from amphibian-like animals during the Carboniferous Period, ~359-299 million years ago (Fig.
1). Invertebrate and non-amniote vertebrate animals had been laying eggs for millions of
years prior to the Carboniferous, but the amniotes did things differently. For
one thing, amniotes reproduce through internal fertilization and lack a larval
stage in their development.
But what really sets amniotes apart is their semi-permeable egg. Amniote eggs
are larger and the yolk and albumin ("egg white") provide nutrients
and water, respectively, to the embryo. One or more membrane layers cover
these components and permit the transfer of gases between the embryo and
the environment (i.e., oxygen passes into, and carbon dioxide passes out of,
the egg). Additionally, the mineralized shell protects the embryo from damage
and desiccation, while still permitting gas transfer.
This evolutionary innovation opened the door to new habitats.
Amniotes did not have to lay their eggs in water like their ancestors  they
could become fully terrestrial and
exploit all terrestrial niches.

Amniotes include reptiles, birds, and mammals. It may seem surprising that mammals
are amniotes  we
don't typically think of them as egg-layers. However, early mammals laid eggs,
and some modern mammals called monotremes (like
the duck-billed platypus and echidna) still do. All other mammals (the placentals and marsupials)
and some reptiles have lost the calcified shell and the female retains the egg during development, resulting in live birth. Most egg-laying amniotes produce eggshell calcium
carbonate in the form of calcite (CaCO3);
however, turtles, unlike all other amniotes, build their eggs out of aragonite
(CaCO3 +
magnesium). Organic material
is deposited simultaneously with the calcium carbonate (Fig. 2). Pore canals
run through the shell and permit gas exchange between the embryo and atmosphere.

The calcareous shell unit is the most basic building block of an amniote egg (Fig. 3). The genetically controlled characteristics of a shell's organic material and calcium carbonate determine a shell's physical properties (structure, rigidity, breaking strength). Many characteristics of the calcium carbonate structure are preserved in fossilized eggshell, but organic matter is not commonly preserved because it decays easily and is usually lost during fossilization. However, in the past few decades, scientists have begun to identify organic matter from eggshell in the fossil record.

The identity of the egg-laying animal can only be absolutely known if embryonic remains are preserved
within an egg. Because shell unit structures are unique among taxa,
they can be used to help in fossil eggshell identification and classification.

Physical properties of the amniote egg
Eggs can be divided into three general categories based on the physical properties
of the eggshell: soft, flexible, or rigid. Eggshell rigidity is determined by
the proportion of inorganic to
organic matter. Soft and flexible eggshell contains more organic matter than calcareous crystalline
material; conversely, rigid eggshell has more calcium carbonate (calcite or aragonite)
than organic matter incorporated into its structure.

Soft eggshell (Fig. 4): Most lizards, snakes, and tuataras lay soft eggs composed of an organic framework and poorly organized calcite crystals. These eggs collapse and shrivel after the animal hatches, and are therefore unlikely to be identified or even preserved in the fossil record.

Flexible eggshell (Fig. 5): Many amniotes, including some lizards, snakes, and turtles, lay eggs with flexible shells. These shells differ from soft shells because of their higher mineral content. Nevertheless, preservation of flexible eggs is also rare in the fossil record.

Rigid eggshell (Fig. 6): Some turtles and geckos, and all crocodilians, dinosaurs, and birds lay eggs with rigid eggshell. The calcite crystals form a relatively thick eggshell of interlocking shell units. Fossilization is more likely to occur in rigid eggshell because the crystalline calcium carbonate (calcite or aragonite) layer is stronger, more durable, and does not shrivel upon hatching.